This invention relates to temperature compensating devices for compensating the effect of temperature changes in an electrical or electronic circuit. In particular, it relates to a temperature compensating device using embedded columnar thermistors for enhanced performance.
Temperature compensating devices are important components in a wide variety of electrical and electronic circuits such as high frequency communication circuits. Communication circuits are typically constructed using components, such as semiconductor devices, whose properties change with temperature. For example, solid state amplifiers are made using semiconductor components, and the current carrying ability of these components decreases with increasing temperature, reducing the gain of the amplifier. In the absence of compensation, such temperature-induced changes can deteriorate the performance of the circuit.
One method for compensating temperature-induced changes in a communication circuit is to cascade the circuit with a temperature compensating device whose pertinent characteristics vary oppositely with temperature. For example, an amplifier can be cascaded with a compensating device that increases in gain with increasing temperature. The cascaded combination minimizes gain variation with temperature.
U.S. Pat. No. 5,332,981 issued to the present applicant and John Steponick on Jul. 26, 1994, and is incorporated herein by reference. The ""981 patent, which is entitled xe2x80x9cTemperature Variable Attenuator,xe2x80x9d describes a passive temperature compensating device using at least two different thermistors which are deposited as films on a substrate. The temperature coefficients of the thermistors are different and are selected so that the attenuator changes at a controlled rate with temperature while the impedance of the attenuator remains substantially constant.
Difficulties with the ""981 device arise because the thermistors are formed as thin, relatively large area films on the surface of a substrate. The films are unduly susceptible to changes in air temperature. Moreover, there can be substantial temperature gradients between the film-air interface and the film/substrate interface. As one consequence, forced air cooling can vary the thermistor temperature and produce unwanted gain ripple. Another difficulty is that the relatively large area of the film requires a relatively large substrate. This increases cost, consumes board space, and degrades high frequency performance. Accordingly there is a need for improved temperature compensating devices.
In accordance with the invention, a temperature compensating device comprises one or more columnar thermistors embedded within a substrate. Because the thermistors are substantially covered by the substrate, they are less susceptible to changes in air temperature and to temperature gradients. Moreover, within the substrate the thermistors can be made thicker and smaller in lateral area, permitting more compact, less expensive devices that exhibit improved high frequency performance. The devices can advantageously be fabricated using the low temperature co-fired ceramic (LTCC) process.